1. Field of the Invention
The present invention generally relates to a method and system of heart rate variability analysis, and more particularly to a method and system of auditory evoked heart rate variability analysis for evaluating the reaction of human autonomic nervous system to sound stimulation and the function of autonomic nervous system.
2. Description of the Related Art
Human autonomic nervous system controls the physiological function relevant to life maintenance through the whole body, which comprises blood pressure, heart rate, tracheal resistance, perspiration, body temperature, and metabolism. These nerve operations can process automatically. Autonomic nervous system mainly includes sympathetic nerves and parasympathetic nerves. Generally speaking, the former is related to resist the environment, whereas the latter is related to propagation. For example, when a person is excited, the former will increase the blood pressure and dilate the pupils while the latter will cause gastrointestinal secretion and genital erection. In general, sympathetic functions and parasympathetic functions are active in young persons, but rather inactive in old persons; in males, sympathetic functions prevail but parasympathetic functions yield; conversely, parasympathetic functions excel sympathetic functions in females. So we can know that sympathetic nerves and parasympathetic nerves are closely related to the daily operation of a human body. Autonomic imbalance may induce various acute and chronic diseases, for example, heart disease, hypertension, etc., and may even lead to a sudden death, if serious. Hence, the protection for autonomic nervous system is not only an important issue in medicine but also a personal concern to an individual everyday. If we can control the function of autonomic nervous system efficiently, such as changing daily schedule, regulating respiratory frequency, and even processing the movement, relaxation, and reducing movement of the eyes, the diseases can be prevented or improved. Therefore, it will be an important regimen. The strong and weak of the sympathetic function are not only related to diseases, but also related to the variation of sleep, and alertness and concentration while awake. Detecting sympathetic function efficiently is helpful to improve the alertness and concentration while awake, or to decrease the alertness and concentration before sleep in order to promote falling asleep. In addition, controlling sympathetic function is also a common used method in psychology.
In recent years, plenty of new technologies to evaluate the autonomic functions were successfully developed. Given the sophisticated computer hardware and software know-how available, today it is possible to detect and perform quantitative analysis of a person autonomic cardiac activity in light of the minute fluctuations of hear rate, known as heart rate variability (HRV), taken while the person is at rest. In other words, the new technologies allow a user to analyze or evaluate a normal person's autonomic functions without interfering with the person's daily life. Researchers discovered that the minute fluctuations of heart rate variability, which can be represented by total power (TP), can be divided into two groups by frequency, that is, high-frequency (HF) component and low frequency (LF) component. The HF component is according to the rhythm of breath, so it is also known as breathing component. The source of the LF component is relevant to vascular motion or baroreflex.
Many physiologists and cardiologists believe that the HF component or total power reflects parasympathetic functions, whereas the ratio of LF component to HF component (LF/HF) reflects sympathetic activity. For instance, patients diagnosed with intracranial hypertension usually have relatively low heart rate variability. The public health investigation of American Framingham found that the death rate of an elder whose LF component of heart rate variability decreases by a standard deviation is 1.7 times that of normal person. Nowadays, a series of software and hardware, which can process spectrum analysis directed against various physiological signals on-line, have been developed. If the LF components of heart rate and blood pressure are the index of depth of anesthesia, for example, it can be found in the intensive care unit that the survival rates of patients decrease while heart rate variability decreases and the LF component of heart rate variability vanishes in a brain-dead person. Furthermore, there are changes in heart rate variability in a patient who exhibits rejection reactions after heart transplantation. However, the autonomic functions differ from person to person. The individual variation affects a lot. Sex, age, the variation of day and night, and etc. can result in the change of the autonomic functions. Moreover, the sounds also result in changes of autonomic nervous function, such that a great sound of more than 100 dB results in a startle reflex, and the long-term exposure to noise also increases the blood pressure. In addition, there are some clinical diseases, such as melancholia, migraine, tinnitus, and vertigo sensitive to sounds, which only can be estimated by the subjective feeling of patients or questionnaires instead of objective methods up to present.
According to the above problems, there is needed to provide an easy operating and sensitive method and system of auditory evoked heart rate variability analysis, which stimulate the auditory system by a sound and then measure the differences of heart rate variability before and after stimulation. The heart rate variability evoked by sounds, which individual variation is less and more sensitive, can predict or evaluate some specific diseases, especially some diseases which are sensitive to sounds. It can also estimate healthiness of autonomic nervous system.